The catalytic production of dimethyl ether (DME) from methanol by catalytic dehydration has been known for many years. The U.S. Pat. No. 2,014,408 for example describes a process for the production of DME from methanol on catalysts such as aluminum oxide, titanium oxide and barium oxide, with temperatures of 350 to 400° C. being preferred.
Further information on the prior art and on the current practice of the production of dimethyl ether can be found in Ullmann's Encyclopedia of Industrial Chemistry, Sixth Edition, 1998 Electronic Release, keyword “dimethyl ether”. In Chapter 3 “Production” it is explained in particular that the catalytic conversion of evaporated methanol usually is carried out in fixed-bed reactors.
From the point of view of reaction engineering, fixed-bed reactors preferably are used for the catalytic dehydration of methanol to DME in the gas phase, since they are characterized by constructive simplicity. The German laid-open publication DE 3817816 for example describes a process integrated in a methanol synthesis plant for producing dimethyl ether by catalytic dehydration of methanol without previous separation of the synthesis gas not converted in the methanol reactor. As dehydration reactor a simple fixed-bed reactor is used.
The dehydration of methanol to dimethyl ether according to the reaction equation2CH3OH=(CH3)2O+H2O.is an exothermal equilibrium reaction. Hence it follows that from a thermodynamic point of view high degrees of conversion are achieved at reaction temperatures as low as possible. On the other hand, from a reaction-kinetic point of view a minimum reaction temperature is required, in order to ensure sufficient reaction rates and thus acceptable methanol conversions. In the literature, this minimum reaction temperature also is referred to as starting temperature or light-off temperature and is dependent on the type and quality of the catalyst used, but also on the definition of an acceptable minimum conversion. During start-up of a DME synthesis reactor, it hence is required to first set this minimum temperature. As soon as the dehydration of methanol has started by forming DME, the minimum temperature can be maintained due to the exothermal release of heat or the reactor temperature even can rise further. Therefore, it often is advisable to equip the fixed-bed reactor with additional cooling devices, in order to avoid too high reactor temperatures. The same on the one hand can damage the catalyst and on the other hand can lead to the formation of by-products, such as carbon monoxide CO, carbon dioxide CO2, hydrogen H2 and methane CH4. The formation of these by-products is undesirable, since they impair the purity of the reaction product and reduce the selectivity of the reaction to DME.
To adjust the above-described light-off temperature or starting temperature, it so far has been common practice to charge the fixed bed of the dehydration catalyst arranged in the reactor interior with a tempered inert gas stream, in order to heat up the catalyst bed by means of convective heat transfer. The heat-up temperature must be controlled carefully, in order to avoid a damage of the catalyst or sensitive plant sections due to an excessive temperature. Furthermore, just like in the synthesis operation of the reactor, a minimum flow velocity of the inert gas through the reactor should not be fallen short of, so that a uniform traversal and hence heat-up of the catalyst bed is ensured.
As inert gas, nitrogen frequently has been used so far, which in turn has been heated up by heat exchange for example against superheated steam. In DME synthesis plants integrated into a plant complex, which also includes an air separation unit, a large nitrogen stream can be provided for a short period without great expenditure. For a large DME synthesis plant, up to 100000 mN3/h of nitrogen must be provided for a short time, when the nitrogen is passed through the synthesis reactor in straight passage when heating up the catalyst bed. Because of the poor heat transfer between the dry inert gas and the catalyst bed, this large amount of nitrogen must be heated up to a temperature above the starting temperature to be adjusted. For this purpose, a correspondingly large heat exchanger must be provided, which correspondingly increases the apparatus expenditure.
When no air separation unit is available, the provision of large amounts of nitrogen is difficult. Alternatively, the heated inert gas stream then can also be guided in a cycle, for which purpose however an additional cycle compressor is required. Since the trouble-free operation of the DME synthesis plant often can last a few years, an additional expensive and rarely used apparatus thus must be provided.